System for exhaust gas treatment comprising a gas ionizing system with ionized air injection

ABSTRACT

The invention proposes a system ( 16 ) for treating the exhaust gases (G) of a motor vehicle combustion engine ( 10 ), particularly a lean-burn diesel engine or engine, comprising a burnt gas (G) exhaust circuit ( 14 ), of the type in which the exhaust circuit ( 14 ) comprises a burnt gas (G) ionization system ( 22 ), characterized in that the exhaust circuit ( 14 ) comprises an ionized air injection system ( 24 ) upstream and/or downstream of the burnt gas (G) ionization system ( 22 ).

The invention proposes a system for treating the exhaust gases of amotor vehicle combustion engine, particularly a lean-burn diesel engineor petrol engine, comprising a burnt gas exhaust circuit, of the type inwhich the exhaust circuit comprises a burnt gas ionization system.

The regulations governing vehicle emissions are primarily concerned withfour types of pollutant: unburnt hydrocarbons HC, carbon monoxide CO,nitrogen oxides NOx and particulates.

In the case of an engine running with an excess of oxygen, that is alean-burn petrol engine or a diesel engine, the unburnt hydrocarbon HCand carbon monoxide CO emissions are reduced by using an oxidationcatalyst, which converts them to carbon dioxide CO₂ in largeproportions.

This oxidation reaction is all the more efficient the higher thecatalyst temperature. The catalyst is accordingly placed as close aspossible to the outlet of the combustion chamber of each enginecylinder.

The nitrogen oxides NOx can be treated by means of a nitrogen oxide NOxtrap called a “NOx-trap”.

In the case of a lean-burn engine, the use of the nitrogen oxide trap isconditioned by the possibility of locally increasing the fuel-air ratioof the exhaust gases passing through the trap.

Today, however, these systems for treating nitrogen oxides Nox andunburnt hydrocarbons HC are still used very little, because theirefficiency is not optimal in all driving conditions. Moreover, theircost is not negligible and their use causes substantial additional fuelconsumption.

To contend with the lack of efficiency, research is currently underwayon coupling these systems with the non-thermal plasma technology.

The technique consists in forming metastable species, free radicals andhighly reactive ions, by collision between the gas molecules and thehigh-energy electrons produced by a discharge, and without raising thetemperature of the medium.

This discharge is obtained by applying high-voltage high-frequencysignals across two electrodes whose geometric configurations may bedifferent. In the so-called lean-burn engine exhaust gases, suchdischarges alter the composition of the gas mixture by promotingreactions such as the oxidation of nitric oxide NO to nitrogen dioxideNO₂, the formation of partially oxidized hydrocarbons from the unburntresidues, and finally, oxidation reactions causing the activation of theparticulates.

Combined with a catalytic post-treatment system for reducing nitrogenoxides NOx, these plasma-generating discharges, which are locatedupstream of the catalyst or in the catalyst, serve to obtain higherreduction rates than those obtained with a catalyst alone, and in a muchwider range of temperatures.

Combined with a particulate filter, the plasma-generating dischargesfacilitate the reaction combustion soot, which is necessary toregenerate the filter media. In fact, the formation of oxidizingactivated species, such as ozone O₃ and nitrogen dioxide NO₂, and ofreducing species, such as partially oxidized hydrocarbons and activatedsoot, promotes the initiation of particulate oxidation at lowertemperature.

To ensure ever higher reduction efficiencies, it is first necessary toobtain an optimal conversion of nitric oxide NO to nitrogen dioxide NO₂.

In general, two main reactions lead to the oxidation of nitric oxide NOto nitrogen dioxide NO₂:

-   -   NO+O+M →NO₂+M, where M=N₂ or O₂ and    -   NO+O₃ →NO₂+O₂

When non-thermal plasmas are used as “ozonizer” via the treatment of theambient air, it is possible to produce up to 50 g of ozone O₃ per kWhconsumed. In this case, the atomic oxygen O formed in the ambient air isfully available for the formation of ozone O₃.

Competing reactions to the production of ozone O₃, such as nitric oxideNO production reactions, only significantly occur when the power inputsemployed are sufficient to introduce a temperature rise that favours thekinetics of formation of nitric oxide NO. The parameters limiting ozoneproduction are the temperature (ozone O₃ is thermodynamically unstableat above 600-650 K) and the steam (moisture content).

In the publications “NO Oxidation Process in Dielectric BarrierDischarge using Multipoint-to-plane Electrodes” and “NOx removal fordiesel Engine exhaust by ozone injection method” presented at theconference on “Non-thermal plasma technology for pollution control” inApril 2001 in South Korea, it was proposed to inject air ionized by anon-thermal plasma, that is to say air that contains a highconcentration of ozone O₃ in the exhaust gases.

Although such methods help significantly to reduce the quantity ofnitrogen oxides NOx present in the exhaust gases, their efficiency islimited to about 60%.

It is an object of the invention to improve the treatment of thenitrogen oxides by non-thermal plasmas.

The invention therefore proposes a treatment system of the typedescribed above, characterized in that the exhaust circuit comprises anionized air injection system upstream and/or downstream of the burnt gasionization system.

According to other features of the invention:

-   -   the ionized air injection system comprises means for ionizing        the ambient air that convert a portion of the oxygen present in        the ambient air to ozone;    -   the air ionization means and the burnt gas ionization system        each consist of at least one reactor of the non-thermal        plasma-generating discharge type;    -   the burnt gas ionization system comprises a plurality of        reactors arranged in series which successively ionize the burnt        gases;    -   the various reactors are separate compartments of a single        vessel;    -   the exhaust circuit comprises a catalyst for treating nitrogen        oxides, that is positioned downstream of the ionized air        injection system.

Other features and advantages of the invention will appear from readingthe detailed description that follows, with reference to the figuresappended hereto, in which:

FIG. 1 is a schematic representation of a combustion engine exhaust linethat comprises a treatment system according to the invention;

FIG. 2 is a schematic representation of the treatment system shown inFIG. 1;

FIGS. 3 to 5 are similar views to that of FIG. 2 showing variants of theinvention.

In the following description, identical, similar or analogous elementsare designated by the same reference numerals.

FIG. 1 shows an internal combustion engine 10 that is prepared accordingto the teachings of the invention.

The engine 10 here is of the lean-burn diesel type or petrol type, thatis, it runs with an excess of oxygen relative to stoichiometricconditions.

The engine 10 comprises an inlet gas intake circuit 12 and a burnt gas Gexhaust circuit 14.

The exhaust circuit 14 comprises a pollution control device 16, thattreats the burnt gases G in order to limit the release of pollutantsinto the air.

The pollution control device 16 comprises a system 18 for treating theburnt gases G, that serves to convert the nitric oxide NO to nitrogendioxide NO₂. Downstream of the treatment system 18, the pollutioncontrol device 16 comprises a catalyst 20 which treats the nitrogendioxide NO₂ to convert it to nitrogen N₂ and oxygen O₂, which are thenatural components of air.

As shown in FIG. 2, the treatment system 18 comprises a burnt gas Gionization system 22 that consists of a reactor 32 of the non-thermalplasma-generating discharge type.

This reactor is used to oxidize the nitric oxide NO to nitrogen dioxideNO₂. The nitric oxide NO is oxidized to nitrogen dioxide NO₂ directly orindirectly via atomic oxygen. In fact, as it happens, as confirmed byall studies conducted on the subject, this conversion of nitric oxide NOto nitrogen dioxide NO₂ cannot be complete and actually tends towards anasymptotic limit, in particular because the nitrogen dioxide NO₂ reachesconcentrations such that the reduction reaction

-   -   NO₂+O →NO+O₂ neutralizes the initial oxidation reaction.

For this purpose, and according to the invention, the treatment system18 comprises an ionized air injection system 24 that is arranged heredownstream of the burnt gas G ionization system 22. However, in avariant (not shown) of the invention, the ionized air injection system24 is arranged upstream of the burnt gas G ionization system 22.

This ionized air injection system 24 comprises an air ionization system26 that consists of a reactor of the non-thermal plasma-generatingdischarge type.

The plasmas produced in these reactors 22, 26, 32 are so-callednon-thermal plasmas, generated by discharges of the “corona discharge”type. They are produced across electrodes that are bare or covered withdielectric barriers of various configurations ranging from parallelplanes and in this case with at least one dielectric barrier, togeometries with a strongly heterogeneous applied field(multipoint-to-plane, coplanar wire or screw-to-plane, coaxial wire orscrew-to-cylinder, etc.).

The inter-electrode spacing (defined as the distance between electrodesin the absence of dielectric, between the electrode and the dielectricin the presence of a single dielectric barrier, between dielectrics inthe presence of two dielectric barriers), may be identical or differentfor each of the reactors and, moreover, variable according to thetreatment conditions (gas throughput to be treated, for example).

The gaseous effluents, injected at atmospheric or different pressure,can flow perpendicular or parallel to the plasma. Finally, depending onthe geometry adopted, the power supply of the reactors 22, 26, 32, whichcan be common or different for each reactor 22, 26, 32, supplies avariable voltage that may be DC, pulsed or AC.

The ionized air injection system 24 comprises an ambient air intakecircuit 28 which connects the reactor 26 to the ambient air via an airfilter (not shown).

According to the invention, the reactor 26 converts the oxygen O presentin the air to ozone O₃ and it is able to produce about 50 g of ozone O₃per kWh consumed.

The ionized air that contains the ozone O₃ produced is then injectedinto the exhaust circuit 14 via an injection line 30 downstream of theburnt gas G ionization system 22, so that the ozone O₃ produced reactswith the nitric oxide NO initially present in the burnt gases G, whenthe ionized air injection system 24 is arranged upstream of the burntgas G ionization system, or with the residual nitric oxide NO present inthe burnt gases G when the ionized air injection system 24 is arrangeddownstream of the burnt gas G ionization system 22.

The treatment system 18 further comprises means (not shown) forcontrolling the air ionization reactor 26 to produce the quantity ofozone O₃ necessary for the conversion of all the nitric oxide NO.

This makes it possible to convert all the nitric oxide NO present in theburnt gases G.

According to a first variant of the invention shown in FIG. 3, the gasionization system 32 comprises a plurality of reactors 32, three innumber here, which are arranged in series and which successively ionizethe burnt gases G.

Since the burnt gases G are ionized several times, the quantity ofnitric oxide NO converted to nitrogen dioxide NO₂ is close to theasymptotic limit value defined above, so that the ionized air injectionsystem only needs to produce a reduced quantity of ozone O₃.

According to a preferred embodiment of the invention shown in FIGS. 3and 5, the various reactors 32 are compartments of a single vessel whichare separated from one another by walls 34, which may or may not begastight.

According to a second variant of the invention shown in FIGS. 4 and 5,the two reactors, that of the gas ionization system 22 and that of theair ionization system 26, are two compartments of a single vessel whichare separated from one another by a wall 34 that is gastight.

According to this variant, the gas ionization system 22 may compriseonly a single reactor 32, as shown in FIGS. 2 and 4, or a plurality ofreactors 32 arranged in series, which are themselves compartments of asingle vessel.

Such an arrangement allows a reduction in the total volume of thetreatment device 18, which then consists of a single vessel.

Whatever the embodiment of the treatment device 18, the various reactors26, 32 are supplied with electricity by a single power supply or byhigh-voltage power supplies that may be identical or different,depending on the function of the reactor 26, 32 with which they areassociated.

1. System (16) for treating the exhaust gases (G) of a motor vehiclecombustion engine (10), particularly a lean-burn diesel engine or petrolengine, comprising a burnt gas (G) exhaust circuit (14), of the type inwhich the exhaust circuit (14) comprises a burnt gas (G) ionizationsystem (22) and an ionized air injection system (24) upstream and/ordownstream of the burnt gas (G) ionization system (22), which comprisesmeans (26) for ionizing the ambient air that convert a portion of theoxygen present in the ambient air to ozone, and of the type in which theair ionization means (26) and the burnt gas (G) ionization system (22)each consist of at least one reactor (26, 32) of the non-thermalplasma-generating discharge type, characterized in that the burnt gas(G) ionization system (22) comprises a plurality of reactors (32)arranged in series which successively ionize the burnt gases (G). 2.Treatment system (16) according to claim 1, characterized in that thevarious rectors are separate compartments of a single vessel (18). 3.Treatment system (16) according to claim 1, characterized in that theexhaust circuit (14) comprises a catalyst (20) for treating nitrogenoxides, that is positioned downstream of the ionized air injectionsystem (24).
 4. Treatment system (16) according to claim 2,characterized in that the various rectors are separate compartments of asingle vessel (18).